Grignards are interesting beasts, particularly with respect to precipitates. There is a Schlenk equilibrium between the alkylmagnesium halide on one side and the dialkylmagnesium plus magnesium dihalide on the other:
2 RMgX <----> R2Mg + MgX2 (that should be an equilibrium arrow)
The actual behavior can be far more complex as dimers and higher oligomers are likely to form in more concentrated solutions, some of which will also have limited solubility, particularly in less polar solvents. And, of course, some of these species will have solvent molecules coordinated as well.
The presence of precipitate in a Grignard does not mean it is "bad" per se. However, all Grignard solutions should be titrated to determine the actual molarity before use. There are routine literature methods for such titrations, and they are easily done on a very small scale. But yes, Tilak is right, if you see an unusually heavy amount of precipitate this suggests that the bottle has aged, and the precipitate may be from reaction with oxygen/water and/or loss of solvent, at which point starting fresh is probably better from a synthetic standpoint.
Best wishes,
Rob Toreki
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Generally, Grignard and pyrophoric reagents such as tert-BuLi should be used/consumed ASAP. These reagents are not good for long term storage purpose. If you see lot of solid sitting in the bottom of reagent bottle, definitively they are not useful for the reaction.
Procuring less volume of these sensitive reagents is always plus.
Tilak
I strongly recommend the chapter on Peroxide-Forming Chemicals in the ACS Handbook of Chemical Health & Safety. I prepared a talk on peroxidizables for our chemistry department a couple years ago and found it invaluable (both the main text and the references).
I'm leery of accepting the argument that Grignard reagents and other reactive compounds contained in the peroxidizable will necessarily suppress the formation of peroxidizables without any actual evidence. If the compound in question is a free-radical scavenger, I get it, but while Grignards sometimes form via a free-radical mechanism, I've never heard of them being considered scavengers or stabilizers.
Another consideration is that the peroxides may form a different phase than the ether or THF, and sometimes the critical concentration for that phase behavior is quite low. This is the source of the crystals sometimes seen floating in bottles of isopropyl ether. So your other compounds may not be relevant because they can't reach the peroxides to degrade them.
A Sigma-Aldrich SDS for methlymagnesium bromide in solution (diethyl ether) I just pulled specifically states that it accumulates peroxides.
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Homewood Laboratory Safety Advocate
Krieger School of Arts & Sciences/Whiting School of Engineering
The Johns Hopkins University
We're in discussion with a research group who uses quite a number of materials flagged by our inventory system as being peroxide formers.. Their claim is that certain of their materials will be unlikely to form peroxides because there are other chemicals mixed in with things life THF and ether (for example, methylmagnesium bromide in THF). My thought is that their logic is sound, but I wanted to push the question to a broader group to see if there is a consensus out there.
Karalyn (Karen) Humphrey, Ph.D
Laboratory Safety Program Manager
& Radiation Safety Officer
Department of Environmental, Health & Safety
"… our job in safety is to make the task happen, SAFELY; not to interfere with the work…" Neal Langerman
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